I have a '68 Firebird with a '70 Grand Prix 455 block installed on the original '68 mounts, engine and subframe. If you do have a '71 block, it will probably have the holes, tapped or not, in all five possible mount locations. Mine was, and it bolted right up with no issues simply by using the original firebird mounts on the engine, and the subframe, just in the Firebird positions, which were not the same holes necessarily used in the A and B body Pontiacs. Don't forget the subframe mounts have an additional brace piece.

I understand that the '70 and later early '70's blocks are the one's most frequently found with all 5 mount positions at least drilled in the blocks, and the holes can be tapped if they are not already. I hope you have all the original Firebird mount pieces and remember where they go. They are different from the block pieces shown in your photo, but they should bolt up pretty easily to a '71 block.

There is a fairly flat plate that bolts to the block, the rubber mount is below that, and the bolt from the rubber mount goes through the frame mount, which is braced by a second bolt on piece, at least on the '68.